feat: #46 auto-categorização no import por histórico de descrições

- TransactionRepository.CategoryByDescription: busca a categoria mais
  recente para cada descrição normalizada no histórico do perfil
- ImportService.Preview: pré-preenche category_id em linhas não-duplicatas
  cuja descrição já foi categorizada antes (case-insensitive)
- Duplicatas não recebem sugestão
- 2 novos testes: auto-categorize + duplicate-not-categorized

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

apps/api/internal/repository/transaction.go

@@ -15,6 +15,8 @@ type TransactionRepository interface {
 	IsExternalIDKnown(ctx context.Context, externalID string) (bool, error)
 	BulkInsert(ctx context.Context, rows []model.ImportRow) (int, error)
 	SaveImportLog(ctx context.Context, filename, format string, imported, duplicates, errors int) error
+	// CategoryByDescription returns the most recently used category_id for each normalized description.
+	CategoryByDescription(ctx context.Context, descriptions []string) (map[string]int, error)
 }
 
 type transactionRepo struct{ db *pgxpool.Pool }
@@ -82,6 +84,42 @@ func (r *transactionRepo) BulkInsert(ctx context.Context, rows []model.ImportRow
 	return count, tx.Commit(ctx)
 }
 
+func (r *transactionRepo) CategoryByDescription(ctx context.Context, descriptions []string) (map[string]int, error) {
+	if len(descriptions) == 0 {
+		return map[string]int{}, nil
+	}
+	pid := middleware.ProfileIDFromCtx(ctx)
+
+	normed := make([]string, len(descriptions))
+	for i, d := range descriptions {
+		normed[i] = normalizeDesc(d)
+	}
+
+	rows, err := r.db.Query(ctx, `
+		SELECT DISTINCT ON (LOWER(description)) LOWER(description), category_id
+		FROM transactions
+		WHERE profile_id = $1
+		  AND category_id IS NOT NULL
+		  AND LOWER(description) = ANY($2)
+		ORDER BY LOWER(description), date DESC, id DESC
+	`, pid, normed)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	out := make(map[string]int)
+	for rows.Next() {
+		var desc string
+		var catID int
+		if err := rows.Scan(&desc, &catID); err != nil {
+			return nil, err
+		}
+		out[desc] = catID
+	}
+	return out, rows.Err()
+}
+
 func (r *transactionRepo) SaveImportLog(ctx context.Context, filename, format string, imported, duplicates, errors int) error {
 	_, err := r.db.Exec(ctx, `
 		INSERT INTO import_logs (filename, format, imported_count, duplicate_count, error_count)